1. Field of the Invention
The present invention relates to improved semiconductor devices and more particularly to improved integrated devices which include insulated gate-type field effect transistors and protective diodes for protecting the insulating films between the gate electrodes of the field effect transistors and the semiconductor bodies from electrical breakdown.
2. Description of the Prior Art
In general, since the gate G1 of an insulated gate-type field effect transistor Q1 as shown in FIG. 1 has a high capacitive input impedance, a very small amount of electric charge accumulated on the gate G1 induces a high voltage and sometimes causes the insulating film (usually silicon dioxide) between the gate G1 and semiconductor substrate 1 to break down. Therefore, it has been proposed that a protective diode be formed, i.e. a zener diode, integrally in the semiconductor body 1 and that the diode be connected in parallel with the gate G1 as shown in FIG. 1. It has been believed that the protective diode could prevent the insulating film from breakdown without interfering with the characteristics of the field effect transistor.
According to investigation, however, it has been revealed that since the PN junction J.sub.R of the diode is biased in the forward direction by noise pulses e.sub.N, a bipolar transistor is formed by the region 2 (as an emitter), the substrate 1 (as a base) and the drain region D1 of the field effect transistor Q1 (as a collector) since the junction J.sub.D1 is usually biased in the backward direction. Minority carriers injected into the substrate 1 from the diode region 2 diffuse in the substrate 1 and reach the drain region D1, as shown by the broken line arrow in FIG. 1. The emitter common current amplifier factor .beta. of this transistor structure is in the order of 10.sup.-.sup.3 to 10.sup.-.sup.4 . This factor of the transistor structure is much smaller than those of usual bipolar transistors (in the order of several tens to several hundreds). Therefore, the parasitic bipolar transistor seems to be of negligible value because of the extremely small current amplifier factor.
When an insulated gate-type field effect transistor having a protective diode is used as a control switch for storing an information signal or an input signal in a memory element having very small capacitance, the parasitic bipolar transistor, according to our study, cannot be neglected even if the current amplifier factor .beta. is extremely small as aforementioned. For example, in a semiconductor integrated circuit, such as a dynamic shift register, as shown in FIG. 2, the capacitive memory element C represents the capacitance between the drain D1 of the insulated gate-type field effect transistor Q1 and the semiconductor substrate 1, the capacitance between the gate G2 of the insulated gate-type field effect transistor Q2 and the semiconductor substrate 1, the distributed capacitance existing between the drain D1 of the transistor Q1 and the gate G2 of the transistor Q2, etc. which capacitance is very small, for example, several pF. This capacitor C stores an information signal V.sub.IN applied to the input terminal I.sub.1. In other words, as shown in FIGS. 3a to 3c when clock pulse V.sub.CP is at voltage level -E.sub.2, the transistor Q1 turns to its ON-state and the "1" or unit level voltage (i.e. the voltage -E.sub.1) of the information signal V.sub.IN is stored in the capacitor C. Thereafter, the capacitor C holds the "1" level voltage regardless of the V.sub.IN signal until the voltage -E.sub.2 of clock pulse V.sub.CP is again applied to the transistor Q1 after the zero voltage of V.sub.CP once returns the transistor Q1 to its OFF-state.
Since the zero voltage level part of the pulse V.sub.CP, however, often includes a noise signal e.sub.N which comprises high frequency components of fairly large amplitude, as shown in FIG. 3a, the diode Rec 1 is biased in the forward polarity by the noise signal e.sub.N. Only in this instant, a parasitic bipolar transistor Q3 is established, the collector current i.sub.C flows through the transistor Q3 and the capacitor C in spite of the OFF-state of the transistor Q1. Therefore the stored charge in the capacitor C discharges. Especially in such a memory circuit device as that which uses a capacitive memory element of very little capacitance an extremely small amount of collector current i.sub.C of the parasitic transistor Q3 causes the charge stored in the capacitor C to be reduced considerably since the amount of the stored charge is very small, and it results in misoperation of the memory circuit device.
The curves 9, 18, 25, 37 and 47 in FIGS. 7, 11, 19, 24 and 28 show the minimum operating frequency v. noise signal level e.sub.N characteristics of the circuitry (a half bit dynamic shift register) as shown in FIG. 2, in which the minimum operating frequency f.sub.C is the frequency below which the wave form of the output signal V.sub.OUT is deformed by noise signals, as shown by broken lines in FIG. 3c, provided that in the relationship as shown in FIGS. 3a to 3c the frequency of the information signal V.sub.IN equals one half of the frequency of the clock pulse V.sub.CP and the duty cycle of the information signal V.sub.IN is 0.5. The lower the minimum operating frequency, the more effective is the memory function of the capacitive element C
It can be read from curve 9 in FIG. 7 that when the noise signal level e.sub.N becomes over 0.5 volt the memory function is reduced. The lowering of the memory function causes trouble especially when a dynamic shift register is formed by connecting in series a plurality of unit circuits as shown in FIG. 2. Curves 7, 12, 23, 33 and 45 in FIGS. 6, 10, 18, 23 and 27 show the minimum operating frequency f.sub.c v. noise signal level e.sub.N characteristics of a 16 bit dynamic shift register composed of circuit devices as shown in FIG. 2. It can be seen from curve 7 in FIG. 6 that the minimum operating frequency f.sub.C goes up especially in dynamic shift registers of many bits and the operating frequency in the low frequency region is restricted by noise signals when noise signal levels become over 0.5 volt.